[Dev-luatex] additional argument to kpse.find_file?

Hello, I found that a problem in the mplib package was to find mpost.mem in TeXLive: It cant be found with kpse.find_file(). I can't be found neither with "kpsewhich --format mem --must-exist mpost.mem", but in TeXLive 2008, it can be found with "kpsewhich --format mem --engine metapost mpost.mem". The problem is that I can't pass the "engine" option to kpse.find_file(). Is it is possible to add it? I think it will benefit more packages than only mplib. Thank you in advance, -- Elie